1. Field of the Invention
This invention relates generally to a laser-plasma extreme ultraviolet (EUV) radiation source and, more particularly, to a laser-plasma EUV radiation source that generates a thin sheet of a cryogenic target material or a linear array of closely spaced filaments of a cryogenic target material, where the sheet or the array of filaments has a width that corresponds to a laser spot size providing a desired conversion efficiency and a thickness that corresponds to a laser/target interaction depth.
2. Discussion of the Related Art
Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process, well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths and higher frequencies. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current trend for photolithography light sources is to develop a system that generates light in the extreme ultraviolet (EUV) or soft X-ray wavelengths (13-14 nm).
Various devices are known in the art to generate EUV radiation. One of the most popular EUV radiation sources is a laser-plasma, gas condensation source that uses a gas, typically Xenon, as a laser plasma target material. Other gases, such as Krypton, and combinations of gases, are also known for the laser target material. In the known EUV radiation sources based on laser produced plasmas (LPP), the gas is typically cryogenically cooled to a liquid state, and then forced through a nozzle into a vacuum chamber as a continuous liquid stream or filament. The lowered temperature of the liquid target material and the vapor pressure within the vacuum environment cause the target material to quickly freeze. Cryogenically cooled target materials, which are gases at room temperature, are required because they do not condense on the EUV optics, and because they produce minimal by-products that have to be evacuated by the vacuum chamber. In some designs, the nozzle is agitated so that the target material is emitted from the nozzle as a stream of liquid droplets having a certain diameter (50-100 xcexcm) and a predetermined droplet spacing. Some designs employ sheets of frozen cryogenic material on a rotating substrate, but this is impractical for production EUV sources because of debris and repetition rate limitations.
The target stream is illuminated by a high-power laser beam, typically from an Nd:YAG laser, that heats the target material to produce a high temperature plasma which radiates the EUV radiation. The laser beam is delivered to a target area as laser pulses having a desirable frequency. The laser beam must have a certain intensity at the target area in order to provide enough heat to generate the plasma.
It is desirable that a EUV source has a good conversion efficiency. Conversion efficiency is the measure of the laser beam energy that is converted into recoverable EUV radiation. It has been shown that to provide a good conversion efficiency, the laser beam spot size at the target area must be larger in diameter than the distance the plasma expands during each laser beam pulse. Further, the target size must be larger than the laser spot size in order for the target to intercept all of the laser energy. For targets that are droplets or cylindrical filaments, the desired laser spot size that provides good conversion efficiency is orders of magnitude larger than the depth that the laser beam enters into and interacts with the target material (5-10 xcexcm deep) to generate the plasma. Therefore, as the size of the droplet or filament is increased, more of the target material is unused or unvaporized because of the limits of the laser interaction depth into the droplet or filament.
EUV radiation must be produced in a vacuum chamber. The remaining target material is pumped out of the chamber in its gas phase after it has been exposed to the laser beam. Thus, it is not practical to simply increase the diameter of the droplet or filament to the laser spot size that provides the desired conversion efficiency because of the system mass flow constraints. For example, for a target droplet of 500 xcexcm and a laser pulse of 750 mJ, only 0.2% of the mass of the droplet is used for making the EUV radiation, and the remaining 99.8% of the mass of the droplet is left over as debris that must be evacuated from the chamber. The vacuum pumps of the system set the practical upper limit for the total mass flow of cryogenic target material into the chamber.
In accordance with the teachings of the present invention, an EUV radiation source is disclosed that generates a flowing sheet or a linear array of filaments of a liquid target material that have a width that matches the desired laser spot size for good conversion efficiency and a thickness that matches the laser beam/target interaction depth. In one embodiment, the EUV source includes a reservoir containing a source of a pressurized cryogenic liquid target material, such as liquid Xenon. The reservoir also includes an array of closely spaced orifices. The liquid target material is forced through the orifices into a vacuum chamber as closely spaced liquid stream filaments of the target material. The liquid streams freeze to form an array of frozen target filaments. A laser beam is directed to a target area in the vacuum chamber where it irradiates the sheet of filaments. The width of the laser beam at the target area covers most of the width of the array of filaments to provide a high conversion efficiency. In an alternate embodiment, the orifices are replaced with an elongated slot that generates a frozen sheet of target material having a desired width and thickness.
Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.